1) Field of the Invention
Embodiments of the present invention relate to apparatuses and methods for measuring loads on a friction stir welding tool of a friction stir welding machine and, in particular, for measuring the loads on the tool during operation of the machine.
2) Description of Related Art
Friction stir welding is a process in which a rotating tool is urged into and/or through a workpiece, e.g., to join multiple members of the workpiece in a solid state or to repair cracks in a workpiece. One conventional friction stir welding machine includes a spindle that holds the rotatable tool. The spindle rotates the tool and moves the tool along a desired path through the workpiece. The tool can define a shoulder that is urged against the workpiece during welding and a pin-like portion that extends from the shoulder into the workpiece. In some cases, the tool can also define threads or other contours on its outer surface. As the tool is urged through the workpiece, a continuous weld joint can be formed. For example, during one conventional friction stir welding process, the rotating tool is plunged into a workpiece or between two workpieces by a friction stir welding machine to produce the required resistance force to generate sufficient frictional heating to form a region of plasticized material. The longitudinal axis of the tool is typically held normal to the surface of the workpiece (or at a small angle relative to the normal direction so that the trailing edge of the shoulder is thrust into and consolidates the plasticized material). Upon solidification of the plasticized material, the members of the workpiece are joined along the weld joint. Friction stir welding is further described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference.
The loads or magnitude of forces exerted by the friction stir welding machine for moving the tool through the workpiece must be maintained above a prescribed minimum in order to generate the required frictional heating. The various loads provided between the tool and the workpiece can be affected by the rotational speed of the tool, the rate at which the tool is translated through the workpiece, the temperature of the tool and workpiece, the size and material properties of the workpiece, and the size and geometry of the tool. For example, threads or other contours provided on the outer surface of the tool can affect both the loads experienced between the tool and workpiece as well as the degree of mixing of the material of the workpiece during welding. In some cases, the forces on the welding tool can be significant, and deviations from optimal loading conditions can affect the quality of the resulting weld joints, the speed at which the joints are formed, and the longevity of the welding tool and welding machine.
In one typical welding operation, the welding machine includes an automated controller that moves the tool along a predetermined path through the workpiece. The controller is programmed to provide certain welding parameters, e.g., a predetermined radial load on the tool, a predetermined axial load on the tool, a predetermined rotational speed, and a predetermined speed of translation through the workpiece. That is, the machine can exert a load of a predetermined force (e.g., in pounds) on the tool in the axial direction of the tool toward the workpiece, rotate the tool at a predetermined speed (e.g., in RPM), and move the tool through the workpiece at a predetermined speed (e.g., in inches per second). Due to variations in welding conditions, such as variations in the thickness or material of the workpiece or geometric variations throughout the workpiece and the welding path, the loads that result between the tool and the workpiece can change significantly during a single welding operation. Thus, the optimum axial load, rotational speed, and translational speed may vary throughout the operation. In some cases, the controller can be programmed to change the welding parameters during the operation in an attempt to adjust for variations. Such welding programs can require complex determinations based on the characteristics of a specific workpiece.
In some cases, the machine can also include internal sensors that detect characteristics of the machine that are indicative of the loading on the tool. For example, the sensors may measure a hydraulic pressure in a hydraulic actuation system that moves the tool and attempt to use that pressure to determine a loading condition on the tool. In order to correlate the output of the sensor with the actual loads on the tool, a “static” calibration operation can be performed by urging the tool against a load measurement device so that the output of the sensor can be calibrated with the output of the load measurement device. Such “static” calibration operations are performed with the tool in a non-operational condition. That is, the tool is neither rotating nor welding when urged against the load measurement device. For a machine calibrated in this way, discrepancies generally exist between the true loads on the tool during operation and the loads determined by the machine's internal sensors.
Thus, there is a need for an improved apparatus and method for measuring loads on a friction stir welding tool of a friction stir welding machine. The apparatus should be capable of measuring the loads that are actually applied to the tool and should be capable of measuring the loads during operation of the machine, i.e., while the tool is rotating and/or the tool is being used to perform a welding operation.